This invention relates to a synchronizing circuit for a video disc playback device and, more particularly, to a synchronizing circuit for a video disc playback device which, in performing superimpose such as display of a picture number, is capable of preventing amplification, in a system for producing an internal video signal such as a picture number on the video disc playback device, of distortion on time base in a video signal reproduced from a disc.
In superimposing an internal video signal such as a picture number (hereinafter sometimes referred to as "internal signal") which is produced in a system inside of the video disc playback device (hereinafter sometimes referred to as "internal signal processing system") upon a video signal reproduced from a disc (hereinafter sometimes referred to as "external signal") of a system processing such external signal (hereinafter sometimes referred to as "external signal processing system"), failure in synchronization between the external signal and the internal signal will cause a trouble such as a distortion in a reproduced picture. For preventing such trouble, a synchronizing signal contained in the external video signal is detected and the internal signal is superimposed on the external signal at a timing of the detected synchronizing signal.
In performing production of an internal video signal in digital processing, quantization (digitalization) of time base is required. Since an external signal from a disc contains jitter (distortion on time base), such jitter is amplified in the quantization process to produce an error equivalent to one clock with a result that distortion tends to be produced in a character of an internal signal displayed on a television.
FIG. 2 schematically shows a structure for providing a superimpose function to a conventional video disc playback device.
A reproduced signal from a disc 12 detected by an optical pickup head 10 (in this signal, jitter component has been removed to some extent by a tangential servo performed in the optical pickup head 10) is demodulated to a composite video signal by an FM detection circuit 14 and thereafter a synchronizing signal is separated from the composite video signal by a synchronizing signal separation circuit 16. For digitalizing the separated synchronizing signal, the separated synchronizing signal is applied to a register 18 which is driven by two phase clocks .phi.1 and .phi.2 in which it is quantized on time base. The digitalized synchronizing signal is then applied to a vertical and horizontal synchronizing signal detection circuit 20 in which a vertical synchronizing signal VSYNC and a horizontal synchronizing signal HSYNC are extracted from the synchronizing signal. For controlling rotation of a disc motor 24, a motor control circuit 22 performs a relatively rough control by comparing in phase a rotation detection pulse from a frequency generator 26 directly coupled to the disc motor 24 with a reference clock .phi..sub.hl produced from a crystal oscillator output and also performs a relatively accurate control by comparing in phase the extracted vertical synchronizing signal VSYNC or horizontal synchronizing signal HSYNC with a reference clock .phi..sub.h2 produced from a crystal oscillator output.
A TBC (time base corrector) 25 is a circuit provided for absorbing a minute jitter remaining in the composite video signal and is constructed of an analog circuit such as a variable delay line. A TBC control circuit 27 extracts a color burst signal from an output signal of the TBC 25 and compares in phase the extracted color burst signal with a reference clock .phi..sub.c produced from a crystal oscillator output of 3.58 MHz corresponding to a subcarrier of the color burst and variably controls the delay time of the TBC 25 in accordance with a phase difference between the two signals thereby causing the TBC 25 to absorb the minute jitter in the external signal.
The external signal provided by the TBC 25 is delivered out from a synthesizing circuit 28.
A character production circuit 30 stores an internal signal to be superimposed on the external signal and provides this internal signal at a timing synchronized with the external signal in response to the vertical synchronizing signal VSYNC and the horizontal synchronizing signal HSYNC. The read-out internal signal is synthesized with the external signal by the synthesizing circuit 28 and thereafter is delivered out. In this manner, the character of the internal signal is superimposed on a predetermined position on the picture of the external signal.
The register 18 which connects the external signal processing system with the internal signal processing system and is driven by the two phase clocks .phi.1 and .phi.2 (clocks produced from crystal oscillation output and rising 455 times for example in one horizontal scanning period 1H) receives an input signal (horizontal synchronizing signal) at the clock .phi.1 and outputs it at the clock .phi.2, as shown in FIG. 3.
Accordingly, a quantization error occurs within one clock range of the clocks .phi.1 and .phi.2. In this case, if jitter of the input signal varies centered on .circle.b and within the extent of .circle.a and .circle.c as shown in FIG. 3, the point of change in the input signal exists between a fall point t1 of clock .phi.1 and a fall point t2 of a next clock .phi.lso that the change in the input signal is loaded at clock .phi..sub.la and delivered out at clock .phi..sub.2a and the output signal therefore becomes .circle.d in either case. In this case, therefore, jitter of the input signal is absorbed and the period of the output signal becomes 455 clocks length which is a normal period. If the central position of jitter is exactly halfway between t1 and t2, the jitter margin becomes equivalent to one clock of the clocks .phi..sub.1 and .phi..sub.2 (140 ns corresponding to half period of the subcarrier of the color burst) in peak-to-peak value.
The phase relation between the input signal and the clocks .phi.1 and .phi.2, however, is not centered on .circle.b as described above. If jitter varies centering on .circle.a or .circle.c , even a small jitter component in the input signal is amplified in the output signal with a result that one cycle of the output signal becomes 454 clocks (i.e., shortening) or 456 clocks (i.e., prolonging).
FIG. 4 shows a case in which the input signal has a very small jitter as shown in .circle.a and .circle.b . In the case of .circle.a , the input signal changes before falling of the clock .phi..sub.la so that this change is loaded at the clock .phi..sub.la and delivered out at the clock .phi..sub.2a. In the case of .circle.b , the input signal changes after falling of the clock .phi..sub.la, this change is loaded only at a next clock .phi..sub.lb and delivered out at the clock .phi..sub.2b. Therefore, notwithstanding that the input signal contains only a very small jitter .tau..sub.i, the jitter is amplified to .tau..sub.o in the output signal. This causes the period of the output signal to be varied by 1 clock from the reference 455 clock period to 454 clocks or 456 clocks.
As a result, if the internal signal is read out from the character production circuit 30 by using the horizontal synchronizing signal HSYNC detected in the internal signal processing system and combined with the external signal by the synthesizing circuit 28 as shown in FIG. 2, a slight distortion as shown by A in FIG. 5 in the external signal processing system which corresponds to jitter .tau..sub.i in the input signal is amplified in the internal signal processing system and displayed on the television, and in the case that the internal signal for displaying a vertical straight line is read out from the circuit 30, the line is distorted by one clock relative to the preceding scanning as shown by B in FIG. 5.
Besides, in a case where a main factor of the jitter component remaining is deviation of the disc center, jitter is inverted at each half rotation of the disc (i.e., every field), this causes distortion of one clock so that the vertical line of the picture, in interlaced scanning, is displayed on the television as meandering as shown in FIG. 6 changing its direction every scanning line and therefore distortion becomes conspicuous.
It is, therefore, a first object of the invention to provide a synchronizing circuit for a video disc playback device which, in superimposing an internal signal on an external signal, is capable of preventing amplification in a internal signal processing system of distortion on time base originally occurring in a external signal processing system.
As will be described more fully later, the first object of the invention is achieved by providing a circuit according to which a window is established in a internal signal processing system to include an expected timing of a synchronizing signal of a external signal processing system and, if the synchronizing signal from the external signal processing system has been detected in this window, the expected timing is assumed to be the timing; of the synchronizing signal of the external signal processing system and the internal signal processing system is controlled on the basis of this assumption. In other words, if the synchronizing signal from the external signal processing system has been, detected in the window established in the internal signal processing system, the expected timing is used as the synchronizing signal in the internal signal processing system even if the time at which the synchronizing signal from the external signal processing system has occurred is deviated from the expected timing.
In the circuit according to the invention in which the window is established in the superimpose operation for preventing amplification of distortion on time base in the internal signal processing correction in the internal signal processing system is made only when the synchronizing signal from the external signal processing system has occurred outside of the window.
In the above described synchronizing circuit according to the invention, correction of window must be made in the internal signal processing system if a synchronizing signal of the external signal processing system has occurred outside of the window.
For effecting correction in the internal signal processing system in such a case, it is conceivable to correct the internal signal processing system in such a manner that, when the synchronizing signal has occurred outside of the window, an expected timing of a next synchronizing signal will come at the center of the window (i.e., shifting of the window position).
If the window is shifted in such a manner, however, the synchronizing signal is corrected excessively in a case where the synchronizing signal has occurred outside of the window but adjacent to it (This is particularly so if the window established has a large width.) and, accordingly, distortion on the television becomes conspicuous. Besides, since the window is corrected excessively, the synchronizing signal after correction occurs in an opposite portion of the window so that it is difficult to bring the window to the center of jitter with a result that the correcting operation in the internal signal processing system is performed too frequently thereby causing a periodic distortion in the picture.
It is, therefore, a second object of the invention to provide a synchronizing circuit in a video disc playback device capable of correcting the internal signal processing system with only a small amount of correction when the synchronizing signal of the external signal processing system has occurred adjacently outside of the window whereby distortion in the picture due to the correction is held at a minimum and the window can be readily brought to the center of jitter.
Functions of a video disc playback device include ones of a still picture and a trick play. The still picture function is one for repetitively reproducing data on a single track. The trick play includes, e.g., a forward or rearward shifting of a picture. These functions can be realized by kicking (i.e., performing a track jump) laser beam of the optical pickup head to a preceding or succeeding track.
In the conventional video disc playback device, there has been the problem that track-kick causes color inversion or color tone error in a reproduced picture from the disc for a single scanning line. This problem is caused by the fact that, in a CAV (constant angular velocity) disc of the NTSC system, color bursts of mutually adjacent tracks are different by half cycle (180 degrees) from each other.
Let us assume, for example, a case in which trackkick has been performed halfway in a certain horizontal scanning period as shown in FIG. 14. In a period of time designated by reference character .circle.1 , the color of the picture is produced on a television receiver side on the basis of color burst A of a preceding track and, accordingly, the color of the picture is inverted. Upon shifting to a next scanning line, the reference subcarrier on the television receiver side is reestablished by color burst B and the color inversion is corrected by this re-establishment in a period of time designated by reference character .circle.2 , for the color burst B is different in phase from the color burst A by 180 degrees. Since, however, this re-establishment involves a relatively large change in phase of 180 degrees, it takes some time before this reestablishment is completed and error is likely to occur in the reference subcarrier in an initial stage resulting in error in color tone.
This problem can be solved by correcting time base of the reproduced video signal from the disc each time track-kick is made. More specifically, by advancing or delaying the reproduced video signal from the disc in phase by a predetermined angle, e.g., 180 degrees, of the color burst each time track-kick is made, the color of the reproduced video signal, e.g., in FIG. 14, is made on the basis of the color burst A in the period of time designated by .circle.1 so that no color inversion is produced. As the reproduced video signal is corrected on time base, the color burst B is also shifted in phase by 180 degrees and becomes continuous to the color burst A so that the re-establishment of the reference subcarrier becomes unnecessary. The likelihood of color tone error therefore can be eliminated.
In the synchronizing circuit achieving the above described first object of the invention, however, the following problem arises: In the synchronizing circuit in which the window is provided for preventing amplification in the internal signal processing system of distortion on time base of the external signal processing system, the correction of time base of the reproduced signal from the disc (i.e., an external signal of the external signal processing system) made when the track-kick is performed is totally ignored by the window and therefore is not transmitted to the internal signal processing system. Accordingly, in superimposing an internal signal on an external signal, even when the external signal processing system has really changed by one clock, the internal signal processing system does not follow such change but produces the internal signal to be superimposed on the assumption that the color burst occurs at the same timing as before. This internal signal has a phase error of 180 degrees as viewed from the real color burst with a result that the color of the superimposed character is inverted.
It is, therefore, a third object of the invention to provide a synchronizing circuit to eliminate the above described inconvenience encountered in absorbing the quantizing error by the provision of the window.
As a function of a video disc playback device, it will be convenient if explanation about functions and manner of handling of the playback device or data for demonstration in a shop can be displayed when a disc is not played back.
This function and the above described superimpose function can be performed by reproducing internal video signals from a character generator provided in a video disc playback device. In superimposing an internal video signal on an external video signal or in switching a play mode from reproduction of the internal video signal to the external video signal or vice versa, synchronization between the internal video signal and the external video signal is required. Synchronization of the internal video signal is realized by a clock produced from a crystal oscillator output which is produced inside the disc playback device (i.e., the internal video signal can be displayed even when a disc has stopped) whereas synchronization of the external video signal reproduced from the disc is realized by a synchronizing signal contained in the video signal. Accordingly, synchronization of the internal video signal and that of the external video signal are performed independently from each other. If, however, horizontal and vertical synchronizing signals of the two systems are not synchronized in phase with each other, a character of the internal video signal cannot be displayed on a desired position on the picture. Further, in switching the play mode from the internal video signal to the external video signal or vice versa, flicker in the picture will take place.
It is, therefore, a fourth object of the invention to provide a synchronizing circuit capable of smoothly matching synchronization of the internal video signal with synchronization of the external video signal and thereby preventing disturbances in a picture in switching of the play mode or in performing superimpose.